Algae Back as Quickly as It Left

Elation brought on by last week’s surprising near-disappearance of the cyanobacteria, or blue-green algae, bloom that appeared in Georgica Pond in East Hampton in June was dashed by a dramatic resurgence of the toxic algal bloom on Monday and Tuesday, which in turn was quickly followed by an order-of-magnitude decline.

As it has since July 1, the pond remains closed to the harvesting of shellfish, crabs, and other marine species by order of the East Hampton Town Trustees, who manage many of the town’s waterways and bottomlands on behalf of the public.

Data collected by a monitoring buoy in the southern end of the pond generally measured fewer than 2 micrograms per liter, and often well under 1 microgram per liter, of cyanobacteria from Aug. 10 until Monday, when it began a steep increase, to 14.28 micrograms per liter on Tuesday afternoon. Since then, the toxic algae had fallen to less than 1 microgram per liter but were on a slight upward trajectory, measuring .95 microgram per liter at noon yesterday. Values above approximately 3 micrograms per liter meet the State Department of Environmental Conservation’s threshold constituting a harmful bloom.

Exposure to cyanobacteria at high concentrations can cause nausea, vomiting, diarrhea, allergic reactions, breathing difficulties, or skin, eye, or throat irritation. It can be fatal to children and animals.

The fluctuating but generally low measurements of cyanobacteria in Georgica Pond over the summer come amid intensifying blooms in 10 other East End lakes and ponds, Christopher Gobler of Stony Brook University’s School of Marine and Atmospheric Sciences said on Saturday. These include Hook Pond in East Hampton, Wainscott Pond, Sagg Pond in Sagaponack, Kellis Pond in Bridgehampton, Mill Pond in Water Mill, and Old Town Pond and Lake Agawam in Southampton.

Dr. Gobler has led a water-quality monitoring effort for the trustees since 2013 and is now also working on behalf of the Friends of Georgica Pond Foundation, a pondfront property owners group that formed last year to fight the pond’s environmental degradation.

The trustees “are considering the situation in Georgica Pond to be unchanged, and will be very careful in making any announcements on it in the future,” Francis Bock, their clerk, or presiding officer, wrote in an email on Tuesday. Yesterday, he said that the intense heat of the past week would presumably cause the water temperature to rise, promoting conditions in which the cyanobacteria bloom could thrive.

The trustees, he said, hope soon to open the pond to the Atlantic Ocean, which is typically done on a biannual basis and serves to flush its waters and restore salinity, causing the algal bloom to dissipate. That, however, cannot be done until piping plovers and least terns, federally protected birds that nest along the shoreline, are confirmed to have departed the area, along with their fledglings. The shorebirds’ presence in the spring prevented the April letting of the pond this year.

There has been speculation that the relatively low measurements of cyanobacteria in the past few weeks were a result of the Friends of Georgica Pond Foundation’s effort to remove macroalgae, which releases nitrogen and phosphorus as it decays and is believed to promote the cyanobacteria blooms that have fouled the pond over the last several years.

“We need more information to conclusively say what the cause is,” Dr. Gobler said on Saturday.

The foundation launched an aquatic weed harvester, a motorized craft that removes macroalgae, in May in an effort to quantify its nitrogen and phosphorus content. It has removed several pickup-truck loads of macroalgae, a project that is meant to determine if removing it is effective in combating cyanobacteria. Depending on the amount of macroalgae in the pond, the harvester is to continue operating through this month.

The foundation “is delighted that to date the 2016 field season has produced low toxic blue-green algae levels,” Sara Davison, the group’s executive director, said in a statement issued on Monday, “but it is still too early to draw any conclusions on the reasons for the decline, and there is still time for new harmful algal blooms to develop.”

The statement proved prophetic, given the following day’s dramatic if brief surge. “It is important to take a long view here,” Ms. Davison wrote in an email on Tuesday evening, “and not react to day-to-day spikes. Many factors can affect the cyanobacteria levels,” including weather, pond depth and salinity, and nutrient input. “Many that we don’t understand fully.”